The pipettes at issue here for actuating a syringe serve to dispense the liquid drawn into the syringe in several steps. They are also termed dispensers or repeating pipettes. At the bottom end of a rod-shaped housing, these pipettes have a receptacle for a flange of a syringe cylinder and, in the housing, have a displaceably receiving body with a plunger receptacle for the top end region of a plunger rod of a syringe plunger. The syringe can be inserted with the flange and the end region of the plunger rod through axially-aligned openings in the receptacles. The flange and the end region are held in the receptacles by means for releasably holding that, for example, are designed as spring-loaded grip levers. Furthermore, the pipette has means for displacing the receiving body that make it possible to partially remove the plunger from the cylinder to draw liquid into the syringe, and press the plunger stepwise into the cylinder for the stepwise dispensing of liquid.
DE 29 26 691 C2 and U.S. Pat. No. 4,406,170 describe means for displacing the receiving body in the housing. These comprise a drawing lever that is connected to the receiving body and projects out of the housing through a straight slot for drawing liquid into the syringe by moving the receiving body away from the receptacle. Furthermore, it comprises a toothed rack and pawl apparatus for moving the plunger in steps by a metering lever that can be moved back and forth. A pivotable pawl is mounted on the metering lever. The toothed rack is connected to the receiving body and is arranged within the pivot range of the pawl. An adjustably movable cover more or less covers the row of teeth on the toothed rack to limit the engagement of the pawl in the toothed rack when pivoting the metering lever. The movable cover can be displaced by means of a knob arranged on the pipette housing. The knob is equipped with an eccentric guide curve in which a guide pin of the movable cover engages. Furthermore, the toothed rack is designed with an upwardly extending projection, and the cover has a downwardly sloping rest that engages with the projection when the plunger is far forward, which allows the cover to be moved away from the toothed rack so that it prevents the pawl from engaging in uncovered teeth of the toothed rack. This remaining travel block prevents a residual amount from being dispensed from the syringe that is less than the metered quantity (dispensing volume) to be dispensed in each metering step.
The Multipette® manual pipette by Eppendorf AG is designed according to the above patent.
Developments of the means for releasably holding the syringe are described in EP 0 656 229 B1 and U.S. Pat. No. 5,620,660. EP 1 724 020 B1 and U.S. Pat. No. 7,731,908 B2 describe a development of the holding apparatuses that make it possible to release the syringe from the pipette by single-hand actuation.
EP 0 657 216 B1 and U.S. Pat. No. 5,620,661 describe such a pipette with a sensor for sensing elevations and recesses on the syringe flange of the syringes, and correspondingly designed syringes. The sensor serves to determine the size of the inserted syringe. Electronics determine the amount of the liquid dispensed in each dispensing step based on the set increment. This is shown on a display.
Developments of the means for displacing the receiving body are described in DE 44 37 716 C2, EP 0 679 439 B1 and U.S. Pat. No. 5,591,408. According to EP 0 679 439 B1 and U.S. Pat. No. 5,591,408, a repeating pipette has a constant increment apparatus that sets a constant value for the length of the first step for moving the receiving body for the actuating section of the syringe plunger toward the cylinder receptacle for the syringe cylinder, the value being independent of the setting of the following increments. Play between the pipette and syringe which impairs the metering precision is overcome by means of this constant reverse travel when the receiving body is moved back toward the cylinder receptacle after drawing liquid.
In the Multipette® Plus manual pipette by Eppendorf AG, the cover has an elongated sleeve made of plastic in which a threaded rod is movably arranged. A metal sleeve is glued in the bottom end of the plastic sleeve. The metal sleeve projects from the bottom end of the plastic sleeve and has a catching edge at the bottom below which the thread is exposed for engaging the pawl. In a metering step, the pawl first slides over the metal sleeve that keeps the pawl from engaging in the thread of the threaded rod. Once the pawl passes the catching edge in the metering step, it drops into the thread. For this purpose, the pawl is pressed by means of a spring against the metal sleeve and thread. Due to production tolerances, the pawl may not drop precisely into the base of the thread under the catching edge but rather on a flank or a peak of the thread. When the pawl contacts a flank or the peak of the thread that is arranged above the thread base, the metering lever must be swung further downward slightly until the pawl engages in the thread base and advances the threaded rod and hence the syringe plunger. This causes a metering error because insufficient liquid is dispensed. When the pawl contacts a flank or the peak of the thread which is arranged below the thread base, an insufficient amount of the pivoting movement of the metering lever is used for displacing the threaded rod and hence the plunger. In this case as well, insufficient liquid is dispensed. It can even happen that the pawl slides off the flank or peak of the thread and drops down further into the thread base, or slides several times across the thread which produces a greater metering error, or no liquid is dispensed. The metering error can be overcome by adjusting the threaded rod. For this purpose, the threaded rod is screwed into an adjustment thread in the receiving body for the top end of the plunger rod and provided with a cross hole. By means of a pin inserted in the cross hole, the threaded rod can be screwed into a different axial position relative to the receiving body. This changes the position of the threaded rod when the pawl drops. This adjustment of the assembled pipette is involved. If the metal sleeve is glued too deeply in the plastic sleeve, too much liquid is dispensed, and the displayed number of metering steps cannot be reached. This error must be overcome by exchanging the cover.
With the Multipette®Plus, a metal strip is affixed to the sleeve made of plastic and has a projecting bend in an elastically extendable section, and a leaf spring running parallel to the toothed rack. The metal strip is held on plastic domes, the ends of which are melted on and shaped by ultrasound. At the end, the toothed rack has a cam, the position of which is adjustable in the axial direction of the toothed rack. At the end of the advancing movement of the plunger, the cam contacts the bend which causes the elastically extendable region of the metal strip to be deflected, and the pawl is prevented from engaging further in the toothed rack. This produces the remaining travel block. The cam must be adjusted after the threaded rod is adjusted so that the remaining travel block takes effect precisely when there is an insufficient metered quantity. The production of the cover and the adjustments are involved. In addition, all of the adjustments must be redone when components of the plunger drive are exchanged.
DE 10 2012 011 938 A1 describes a pipette in which the cover has a first cover part made of plastic on which the guide pin is formed. Arranged on the first cover part is a second cover part in the form of a metal strip on which the catching edge is formed and that has the bend and leaf spring of the remaining travel block. The relative position of the first and second cover part to each other in the longitudinal direction of the toothed rack can be adjusted by means of a cam apparatus of the cover. The distance between the catching edge and guide pin can be precisely adjusted therewith to ensure that the pawl drops precisely into the toothed rack despite production tolerances, wear or repair. In adjusting the catching edge, the remaining travel block is also moved so that the remaining travel block must be subsequently adjusted as well by setting the axial position of the cam on the toothed rack. With this cover as well, the production and assembly effort is high since two adjustments always have to be made. When plunger drive components are exchanged, the adjustments need to be repeated. An additional disadvantage is that the cam on the toothed rack is first guided on a smooth inner surface of the cover part until it enters a longitudinal slot in the first cover part upon contacting the bend. When the cam passes into the longitudinal slot, a small lateral movement of the toothed rack may occur that prevents the pawl from correctly dropping into the toothed rack if the adjustments were not performed with sufficient precision.
With the Handy-Step-S manual pipette by Brand, the liquid volume to be metered in each step is also adjusted with a knob. Furthermore, the engagement of the pawl in the toothed rack can be adjusted with the knob. For this purpose, the knob has two knob parts whose angular position relative to each other can be adjusted by a gearing. One knob part has markings for adjusting the metering volume. The other knob part has the guide curve in which the pin engages that projects from the cover. By turning the knob, the cover is displaced along the toothed rack. By adjusting the angular position of the two knob parts, the position of the catching edge can be adjusted so that the pawl drops into the trough of a tooth. With this pipette, the high structural complexity and required complexity in adjusting the assembled pipette is disadvantageous.
The above pipette is basically described in DE 20 2010 010 942 U1.
Against this background, an object of the invention is to create a pipette in which a dropping of the pawl into the toothed rack can be optimized with reduced complexity in production and assembly.